Common techniques for making displays and semiconductor electronic devices involve several imaging steps. Typically, in each step, a substrate coated with a resist or other sensitive material is exposed to radiation through a photo-tool mask to effect some change. Each step has a finite risk of failure. The possibility of failure at each step reduces the overall process yield and increases the cost of the finished article.
A specific example is the fabrication of color filters for flat panel displays such as liquid crystal displays. Color filter fabrication can be a very expensive process because of the high cost of materials and typically low process yield. Traditional photolithographic processing involves applying color resist materials to a substrate using a coating technique such as spin coating, slit and spin or spin-less coating. The material is then exposed via a photo-tool mask and developed.
Laser-induced thermal transfer processes have been proposed for use in the fabrication of displays, and in particular color filters. In such processes, a color filter substrate also known as a receiver element is overlaid with a donor element that is then image-wise exposed to selectively transfer a colorant from the donor element to the receiver element. Preferred exposure methods use laser beams to induce the transfer of the colorant to the receiver element. Diode lasers are particularly preferred for their ease of modulation, low cost and small size.
Laser induced “thermal transfer” processes include: laser induced “dye transfer” processes, laser-induced “melt transfer” processes, laser-induced “ablation transfer” processes, and laser-induced “mass transfer” processes. Colorants transferred during laser-induced thermal transfer processes include suitable dye-based or pigment-based compositions. Additional elements such as one or more binders may be transferred.
Conventional direct imaging systems have employed a limited number of imaging beams. Conventional direct imaging systems have also employed beams having a Gaussian intensity distribution. U.S. Pat. No. 6,242,140 to Kwon et al. describes the use of a laser beam with a uniform energy distribution, or a laser beam which scans by dithering. Other conventional systems have employed hundreds of individually-modulated beams in parallel to reduce the time taken to complete images. Imaging heads with large numbers of such “channels” are readily available. For example, a SQUAREspot® model thermal imaging head manufactured by Kodak Graphic Communications Canada Company, British Columbia, Canada has several hundred independent channels. Each channel can have power in excess of 25 mW. The array of imaging channels can be controlled to write an image in a series of swaths which are closely abutted to form a continuous image.
Even very small variations in the output radiation conditions incident upon the imaged media can cause imaging artifacts, such as banding and rough edges, in laser-induced thermal transfer processes. Variations in the output radiation emitted by the array of imaging channels may originate from channel-to-channel variations of power, beam size, beam shape, focus and beam coherence. Artifacts may not be solely attributable to the imaging system. The imaged media itself may also contribute to banding and other imaging artifacts.
Some prior art multi-channel imaging systems apply calibration methods that adjust the radiation output of all channels in an imaging array to be equal. Other calibration methods operate all channels of an imaging head at once to image a swath and attempt to establish uniformity in recorded optical properties across the swath by adjusting the outputs of channels in the array. EP 434,449A2 and U.S. Pat. No. 6,618,158 describe methods for establishing a uniform power distribution across a multi-channel imaging array or reducing variations between channels of an imaging array.
Image quality is especially important in the production of color filters. Color filters typically have a repeating pattern of spaced-apart color elements (the elements are usually of three colors such as red green and blue). Since the color elements form a repeating pattern, any periodic variations introduced by an imaging process can lead to a visual beating perceptible to the human eye. Rough edges along the boundaries of color elements can lead to colorless voids that further adversely impact the quality of the color filter.
There remains a need for cost effective and practical imaging methods and systems that permit making high-quality images of patterns of features. There remains a need for imaging methods that lessen the visibility of imaging artifacts associated with the imaging of repeating patterns of features, such as the patterns of color elements in a color filter, with a multi-channel imaging head.